This invention relates to a method and apparatus for removing contaminants dissolved in groundwater.
Contamination of the groundwater with potentially hazardous materials is a common problem facing industry, the government and the general public. Frequently, as a result of spills, leakage from storage facilities or surface discharges, contaminants percolate into groundwater, thereby posing a health threat to diking water supplies. While groundwater is not as susceptible to pollution as surface water, once polluted, its restoration is difficult and long term. Various methods for withdrawing and treating contaminated groundwater have met with limited success. Typically, groundwater is removed from the saturated zone, treated and then returned to the saturated zone. This method involves great expense and incurs risks inherent in treating heavy metals and other contaminants, such as nitrates, present in the subsurface.
The method and apparatus of the present invention utilizes the principle of insitu reactive zones for metal precipitation. Precipitation is a process of producing a separable solid phase within a liquid medium. The method may involve installing injection wells into the saturated zone of contaminated soil. A substantially impervious well casing may be placed in the borehole with a fluid-permeable section at its base. Carbohydrates and sulfates may be metered into the conduit under pressure to facilitate proper mixing and dispersion in the saturated zone. Optionally, a mixing pump at the base of the conduit may be utilized to provide a more homogeneous mixture within the conduit. The mixture may then permeate through the fluid-permeable screen of the conduit at its base and mix with the surrounding groundwater. Heterotrophic and sulfate reducing microorganisms indigenous to the soil microflora may then serve as a catalyst for the precipitation process. Two reactions involving the microbes form part of the present invention. The first reaction utilizes the carbohydrates and the dissolved oxygen in the groundwater to form carbon dioxide and water. The result of the first reaction causes a depletion in the oxygen level and leads to the formation of anaerobic conditions in the groundwater. As a result, the sulfates present are reduced to form sulfide ions. These sulfide ions then react with the dissolved heavy metals to form a solid precipitate which eventually is filtered out by the soil matrix. There is no need to remove the precipitate from the soil matrix because it is an insoluble, harmless precipitate.
Instead of injecting carbohydrates and sulfates separately via two injection streams this invention may utilize molasses extract to introduce both carbohydrates and sulfates.
As an example, the following reactions are indicative of the process utilizing sugar with sulfate to precipitate, dissolved lead, Zinc, Mercury and Nickel from groundwater:
C.sub.6 H.sub.12 O.sub.6 +6O.sub.2 6CO.sub.2 +6H.sub.2 O PA1 3SO.sub.4.sup.2- +C.sub.6 H.sub.12 O.sub.6 6CO.sub.2 +6H.sub.2 O+3S.sup.2- PA1 Pb.sup.2+ +S.sup.2- PbS.dwnarw. PA1 Zn.sup.2+ +S.sup.2- ZnS.dwnarw. PA1 Ni.sup.2+ +S.sup.2- NiS.dwnarw. PA1 Hg.sup.2+ +S.sup.-- .fwdarw.HgS.dwnarw.
The use of molasses extract to achieve these reactions is a unique application. The concept of precipitating these metals in an insitu reactive zone rather than in an above ground aqueous phase reactor is also a unique development. The hydrogeological manipulations used in this invention to cause a homogeneous insitu reactive zone in all three dimensions is also unique. Using the soil matrix itself to filter out the insoluble metal precipitates is also unique.
The insitu reactive zone concept can also be applied to microbially denitrify the dissolved nitrates (NO.sub.3.sup.-) to nitrogen gas. The technologies used today to decontaminate dissolved NO.sub.3.sup.- in groundwater involves pumping the contaminated groundwater and using above ground technologies, such as ion exchange beds, reverse osmosis or anaerobic bioreactors. In this invention, injection of carbohydrates alone will create an anaerobic zone due to the depletion of the dissolved oxygen. Then the denitrifying microbial consortia will degrade the NO.sub.3.sup.- ion first to nitrite ion (NO.sub.2.sup.-) and eventually nitrogen (N.sub.2 ) gas. The nitrogen gas, thus formed, will be eventually stripped into the soil gas. Thus, completely removing the dissolved nitrate contamination from the groundwater. EQU C.sub.6 H.sub.12 O.sub.6 +6O.sub.2 .fwdarw.6O.sub.2 +6H.sub.2 EQU NO.sub.3.sup.- .fwdarw.NO.sub.2.sup.- .fwdarw.N.sub.2 .uparw.
The present invention may be practiced utilizing single injection wells or in multiple clusters depending upon the depth of the saturated zone, the geology of the remediation site and the degree of mixing that may be created by each individual injection well. It may be appreciated that required microbial cultures may be added to the soil matrix. This may be required where the indigenous microbes are not present in sufficient numbers to initiate the reactions.
Objects and advantages of the present invention will be readily apparent upon a reading of the following description.